1. Field of the Invention
This invention relates to a radial tire for airplane, and more particularly to an airplane radial tire capable of establishing the improvement of cur resistance in the riding on foreign matter and high-speed durability and the weight reduction of the tire.
2. Description of the Related Art
The radial tire for airplane is defined to satisfy a basic internal pressure as very high as more than 10 atmospheric pressure as an official standard but also the reinforcing member for the tire is required to have a resistance top pressure corresponding to 4 times of the basic internal pressure in order to satisfy the requirement of higher reliability. In the belt of the tire, therefore, the resistance to pressure is satisfied by laminating many plies each comprised of organic fiber cords. On the other hand, it is imposed from airplane makers to satisfy the demand of significantly reducing the tire weight, and hence the establishment between the tire performances and the weight reduction is an important issue in the tire design.
Since the airplane radial tire is generally used under conditions of higher pressure and higher load, there is caused a problem in the performance not damaging the tread when the tire as a whole rides on foreign matters, i.e. so-called envelopability. That is, there is a problem that the resistance force to the foreign matter becomes particularly weak at a state of stretching the tread rubber in the circumferential direction of the tire and the foreign matter easily penetrates into the inside of the tire tread to cause the damage of the tire.
At the present, the belt structure constituted by winding aliphatic polyamide fiber cord (i.e. nylon fiber cord), which is widely used as a belt reinforcing member in the radial tire for airplane, zigzag in the circumferential direction is possible to effectively suppress the occurrence of standing wave during the high-speed running and develops an excellent function as a tire reinforcing structure (see JP-A-H05-193306). On the other hand, since the nylon fiber cord is relatively small in the fracture strength, it is required to arrange many belt reinforcing layers for satisfying the aforementioned resistance to pressure, which obstructs the weight reduction of the tire. Further, since the nylon fiber cord is relatively low in the elastic modulus, it is difficult to suppress the stretching of the tread rubber under a high internal pressure in the use of the tire or the resistance force to the foreign matter is relatively small.
In order to solve this problem, it is examined to use an aromatic polyamide fiber or so-called aramid fiber as a cord material for the belt layer from a viewpoint that it is advantageous to adopt an organic fiber having a high fracture strength and a high elastic modulus. Since the aramid fiber has a fracture strength higher by 1.5-2 times and an elastic modulus higher by 8-10 times than those of the nylon fiber, it is expected to reduce the tire weight while keeping the resistance to pressure and improve the cut resistance by suppressing the stretching of the tread in use.
However, since the aramid fiber cord is small in the fatigue strength to the compression force in the axial direction as compared with the nylon fiber cord, it is confirmed that in case of the zigzag belt structure widely adopted as a belt structure in the radial tire for airplane, the cords are subjected to compression stress in cord bending portions at an end of the belt layer due to strain during the running to cause cord fatigue, which results in the occurrence of troubles such as cord cutting and the like and hence the desired high-speed durability can not be developed.
On the other hand, JP-A-H11-334313 discloses that polyketone fiber cord is used in a belt of a tire for passenger car. However, there is not yet known a technique that such a polyketone fiber cord is applied to the tire for airplane.